# How to Take the Derivative of the Sigmoid Function

Hongtao Hao / 2022-10-30

Given that we have the sigmoid function:

$$\sigma(x) = \frac{1}{1 + e^{-x}}$$

What is its derivative?

It can be rewritten as

$$\sigma (x) = (1 + e^{-x})^{-1}$$

Say that we have:

$$m(x) = -x$$

$$t(m) = e^{m}$$

$$h(t) = 1 + t$$

$$g(h) = h^{-1}$$ Then, we have

$$\sigma(x) = g(h)$$ So:

\begin{aligned} \frac{d\sigma}{dx} & = \frac{dg}{dh}\cdot \frac{dh}{dt}\cdot \frac{dt}{dm}\cdot \frac{dm}{dx} \\ &= (-h^{-2})\times 1\times e^{m}\times(-1) \\& = h^{-2}\cdot e^{m} \\ &=e^{-x}\cdot (1+t)^{-2} \\ &= e^{-x}\cdot(1+e^{m})^{-2} \\ &= e^{-x}\cdot (1+e^{-x})^{-2} \\ &= \frac{e^{-x}}{(1 + e^{-x})^{2}}\end{aligned}

If you play with it, you will know that

$$\frac{d\sigma}{dx} = \frac{e^{-x}}{(1 + e^{-x})^{2}} = \sigma(x)\cdot (1 - \sigma(x))$$

This post is inspired and based upon Taking the derivative of the simgoid function by Danny Denenberg.

#ML